"Methanation" is the name given to a catalytic reaction which yields methane gas from carbon monoxide and hydrogen according to the equation: EQU CO + 3H.sub.2 = CH.sub.4 + H.sub.2 O + heat. (1)
The limited availability of methane from natural sources coupled with the enormous utility of methane as a clean, sulfur-free fuel combine to create a great need for "synthetic natural gas." Methane by methanation now holds great economic significance because the reactants can be obtained by the reaction of the more readily available coal with steam according to the basic equations: EQU 2C + 2H.sub.2 O = 2CO + 2H.sub.2 ( 2) EQU co + h.sub.2 o = co.sub.2 + h.sub.2 ( 3) EQU 2c + 3h.sub.2 o = co.sub.2 + co + 3h.sub.2 ( 4)
since reaction (2) requires heat input, oxygen/air is added to the reactants and the products therefore contain carbon dioxide and nitrogen.
Reaction (1) is highly exothermic and is reversible so that high temperatures tend to reduce the yield of methane. Accordingly, efficient heat removal poses a problem in all methanation processes. Conventional prior art methanation processes are conducted by passing the gaseous reactants through a packed bed of a catalyst which is typically nickel or nickel alloyed with platinum. Such a process is disclosed for example, in U.S. Pat. No. 3,930,812 issued to Harris et al. Harris et al control temperature by injecting liquid water into the reaction zone which removes heat by vaporization. However, the packed bed processes such as that of Harris et al are characterized by a large pressure drop across the reactor.
Dorschner et al, U.S. Pat. No. 2,662,911, approaches the heat removal problem by conducting the reaction in a plurality of catalyst packed tubes, vertically arranged in a water-containing drum. Dorschner, U.S. Pat. No. 2,740,803, discloses methanation in a fluidized bed provided with double-walled, bayonet type heat exchangers through which water is passed (FIG. 4). This latter Dorschner patent also discloses an embodiment (FIG. 1) wherein the catalyst is passed into "contact tubes vertically arranged in a water-containing drum and having diameters which progressively decrease from the top to the bottom." These methods, like the more conventional packed bed methods, are also characterized by high pressure drops across the reactor.
All of the foregoing prior art methanation processes are characterized by the use of a granular or particulate catalyst which tends to generate coke and plug up over prolonged periods of usage.